1. Field of the Invention
The present invention relates to seismic data acquisition. More particularly, the invention relates to the acquisition of seismic data in marine environments.
2. Brief Description of the Prior Art
Marine seismic exploration investigates and maps the structure and character of subsurface geological formations underlying a body of water. Marine seismic data is typically gathered by towing seismic sources (e.g., air guns) and seismic receivers (e.g., hydrophones) through a body of water behind one or more marine vessels. As the seismic sources and receivers are towed through the water, the seismic sources generate acoustic pulses that travel through the water and into the earth, where they are reflected and/or refracted by interfaces between subsurface geological formations. The seismic receivers sense the resulting reflected and/or refracted energy, thereby acquiring seismic data that provides information about the geological formations underlying the body of water.
Typically an array of thousands of seismic receivers is used to gather marine seismic data. The seismic receivers are generally attached to streamer cables that are towed behind the marine vessel. It is known that the relative positions of the marine seismic receivers during seismic data acquisition can affect the quality and utility of the resulting seismic data. However, unpredictable environmental forces such as currents, winds, and sea states present in many marine environments can cause the relative positions of marine seismic receivers to vary greatly as they are towed through the water. Therefore, it is common for steering devices (commonly know as “birds”) to be attached to the streamer cables so that the relative positions (both lateral and vertical) of the seismic receivers can be controlled as they are towed through the water.
FIG. 1 is a simplified depiction of a conventional marine seismic data acquisition system employing a marine vessel 10 to tow seismic sources 12 and a system 14 of steerable seismic streamers 16 through a body of water 18. Each of the seismic streamers 16 includes a streamer cable 20, a series of seismic receivers 22 coupled to the cable 20, and a series of steering devices 24 coupled to the cable 20. As depicted in FIG. 1, during conventional marine seismic acquisition, the steering devices 24 are used to maintain substantially constant lateral spacing between the seismic streamers 16.
A common problem encountered with conventional marine seismic surveys is “gaps” in the acquired seismic data. These data gaps can occur when the spacing between adjacent acquisition passes is too large to provide sufficient resolution for proper data processing. Gaps in seismic data can be caused by a number of factors including, for example, skewing of the seismic streamers relative to the direction of travel of the towing vessel during data acquisition. Even when steerable streamers are employed, gaps in seismic data are common, particularly when strong crosscurrents are present. When strong crosscurrents are present during seismic data acquisition, it is not practical to maintain all the streamers in an orientation that is parallel to the direction of travel of the towing vessel because fighting strong crosscurrents with steering devices produces noise that dramatically reduces the quality of the gathered seismic data.
When gaps in marine seismic data are discovered, the areas corresponding to the data gaps must be resurveyed—a process commonly known as “shooting in-fill” or “in-filling.” Unfortunately, the existence of gaps in marine seismic data may not be discovered until the initial marine seismic survey has been completed and the resulting seismic data is being processed. Obviously, in-filling is very undesirable because of the significant expense and time involved in resurveying in-fill areas that may be located hundreds of kilometers from one another or even retransiting the same vessel pass again to make up coverage.